The incident is important, say lawyers for the detainees, because it highlights the oppressive, punitive character of daily interactions between guards and detainees at the controversial prison camp here.

Military officials, on the other hand, say it illustrates how savvy detainees try to manipulate the system to continue fighting the US behind bars by any means available.

But the Jan. 7 incident may be significant for a different reason: It highlights an absence of independent scrutiny of detainee treatment at the Gauntánamo detention camp and the impossibility under current circumstances of determining who is telling the truth.

On a recent tour by reporters of the Guantánamo camp, questions about the incident were dismissed by military officials with general denials. Requests to see medical records, incident reports, video tapes, and investigation documents were rejected. Officials said surveillance cameras in the exercise area are not equipped with recording devices and thus cannot be reviewed to verify the guards' account or establish Mr. Ismail as a liar or as a bonafide victim.

Reporters are not allowed to interview detainees. Nor are they permitted to conduct unsupervised interviews with guards and other camp personnel. Every interview during a recent two-day tour of the camp was conducted in the presence of at least one, and frequently several, military officials. Some took notes of what was being said and asked.

The Jan. 7 incident is minor compared with the massive policy challenges facing President Obama in trying to fulfill his pledge to close the Guantánamo detention camp within a year. But it does raise a more fundamental issue: There is nothing in the Geneva Conventions specifically mentioning access to the sun or to fresh air.

Common Article 3 of the conventions requires that detainees "shall in all circumstances be treated humanely." That is the essence of the government's duty.

But what does Mr. Obama consider to be humane?

A special Pentagon review panel is expected to release a report – perhaps as early as Monday – assessing whether the 242 prisoners being held here are being held under humane conditions of confinement.

News reports have quoted unnamed sources as saying that although the review panel will suggest a few changes at Guantánamo, it essentially finds that the prison camp for terrorists complies with the demands of the Geneva Conventions.

If these findings are embraced by the Obama administration and set in policy, it would mark a major setback for detainee lawyers and human rights groups who have been fighting to bring a measure of due process into the detention camps. Some are asking for the placement of independent human rights monitors in the camps.

The US Supreme Court has ruled that Guantánamo detainees have a constitutional right to file habeas corpus lawsuits with federal judges in Washington, D.C., to challenge the legality of their ongoing open-ended military detention.

But the high court has declined to address whether similar legal and constitutional protections also apply to the detainees' conditions of confinement.

Two weeks ago, a federal judge threw out a lawsuit charging that the forced feeding of detainees on a hunger strike at Guantánamo amounted to a form of torture. The judge ruled that under the Military Commissions Act of 2006, Congress stripped the courts of jurisdiction to hear cases involving day-to-day treatment of detainees at Guantánamo.

"Many detainees have complained of brutal treatment, lack of medical care, and long placements in solitary confinement," wrote US District Judge Gladys Kessler in her Feb. 10 decision. "To this court's knowledge, none of these allegations, or the government's denials, have been fully tested and subjected to the rigors of cross-examination in open court." She added, "They may never be."

A second federal judge is considering a similar lawsuit filed on behalf of detainee Muhammed Khan Tumani. His suit seeks a court order directing military authorities to move him from Camp 6, a high-security facility, to the less restrictive Camp 4.

Government lawyers say the judge lacks jurisdiction to hear the case since Congress passed the court-stripping measure in the Military Commissions Act.

According to government lawyers, there are no legal avenues open to detainees to invite impartial judicial scrutiny of conditions at Guantánamo.

The Jan. 7 incident involving Ismail's quest to feel the sun would have remained unknown to the outside world were it not for his lawyer, David Remes, legal director of the human rights group Appeal for Justice.

"Yasin hadn't seen the sun in a month," Mr. Remes says. His client spends 22 hours every day alone in a windowless cell in Camp 6. Recreation times are frequently scheduled at night, he says.

When guards refused to move him to the sunny pen, Ismail and the guards exchanged words. At some point, Ismail removed his footwear and flung it against the mesh wall of the enclosure toward the guards standing outside. The guards accused Ismail of attacking them, according to the lawyer's account.

They left him in the closed enclosure throughout the afternoon and into the night. He awoke to the sound of a team of guards in full riot gear entering the cage. Ismail was shackled, then beaten, the lawyer says.

"They blocked my nose and mouth until I saw death, then they let me breathe," Ismail said, according to the lawyer's notes. "They hit me on my ribs until I felt my brains flying."

Remes says a guard urinated on Ismail's head. The next morning, Ismail discovered a bloodstain on his pillow. He was bleeding from his ear, Remes says.

Military officials tell a different story. "It is a complete and total fabrication," said Rear Adm. David Thomas, commander of the Guantánamo detention camp.

According to a camp spokesperson, Ismail "assaulted multiple guards while in the recreation yard by spitting on them and throwing objects at them, including a flip flop and library book."

The military account continues: "He then refused to leave the recreation yard. Later a 'forced cell extraction team' [guards in riot gear] was used to remove the detainee from the pen to return him to his cell."

The guards in riot gear were "passive in nature and used the minimum amount of force necessary," according to the military account. Officials said no guard urinated on Ismail. Medical staff examined both the detainee and the guards and no injuries were observed, they said.

Admiral Thomas said the guards in the riot-gear teams are required to use only the minimum amount of force necessary. "When we do it, it is videotaped and reviewed to ensure nothing untoward happened," he says. "Any alleged injury is documented, photographed."

Thomas says he does not believe the videotapes are maintained as potential evidence. "The detainees know exactly how to make an allegation of abuse, and they know we will investigate," he says. "Every allegation of abuse is thoroughly investigated."

Remes disagrees. "I think the military has proved itself incapable of self-policing," he says.